It is well known in color science that the same pictorial image may have different color appearances if viewed in different environments. In particular, the visual appearance of an image will change if the conditions of its surround (the area immediately adjacent to and surrounding the image) are changed.
Changes in the image surround conditions are commonly encountered in practice. In digital image reproduction, images may be scanned, electronically transmitted, and then viewed in surrounds different from that of the original image. For example, a digital imaging system may create an overhead or 35 mm projection slide from an original reflection image, and the slide may then be viewed in a surround condition quite different from that of the original image. Similarly, an imaging system may produce images on a variety of different output media, each of which is viewed in a different surround condition.
Currently, color-appearance-model parameters are used (via transformations of digital images) for matching the color appearance of the image between different surround conditions. In particular, such parameters are used to account for the fact that an observer's perceived lightness contrast of the image will be influenced by the relationship of the luminance of the surround to the average luminance of the image itself. A surround-parameter having a value of 1.50, for example, would indicate that the perceived lightness contrast of the image will be reduced by a factor of 1/1.50 and that the luminance contrast of the image would have to be raised by a factor of 1.50 in order to compensate for this perceived reduction.
Presently, either of two sets of color-appearance-model parameter values are used to account for such changes in perceived lightness contrast as a function of the surround. The parameter values used are dependent on the relationship of the luminance of the surround to that of the image. The terminology traditionally used to describe this relationship is as follows: the surround is defined as "dark" if the surround luminance is significantly less than that of the image; the surround is defined as "average" (or sometimes as "light") if the surround luminance is greater than or similar to that of the image; and the surround is defined as "dim" if the surround is between "dark" and "average/light." R.W.G. Hunt describes a surround-compensation transformation in The Reproduction of Colour and a color-appearance model in Measuring Colour that include surround parameters having a value of 1.50 for a dark surround, a value of 1.25 for a dim surround, and a value of 1.00 for an average/light surround. Alternatively, U.S. Pat. No. 5,447,811 by Buhr et. al. discloses a surround-compensation transformation having a parameter value of 1.30 for a dark surround, a parameter value of 1.10 for a dim surround, and a parameter value of 1.00 for an average/light surround.
The parameter values in these and other presently known color-appearance models and transformations typically are based on experiments employing simple test stimuli. As a result, the parameter values of presently known and utilized color-appearance models and transformations, while appropriate for such stimuli, may not be appropriate for applications involving more complex stimuli such as pictorial images. Therefore, improved parameter values are needed for such applications in order to achieve better color-appearance matching in different surrounds than possible with existing values.